Don't most already prepared curry powders contain?!

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Not all curry powders are created the same. It changes by region of India and individual cook. Also, some spices are more costly than others and cheaper curry powders will omit or use less of them.

Tumeric is cheap and just about every one uses tumeric. Cumin is more costly but is an important part of most curry powders. Cardamom is one of the most costly components of curry powder and is used sparingly in cheaper brands. Mustard is iffy. Some add it and some don't (not due to cost). Chili powder is also a big variable due to people's tastes

Well, I'd say all curry powders contain tumeric and most contain cumin. The brand I prefer is Chief, and I've never prepared a curry powder from scratch though I have used many recipes that seemed to want me to. Perhaps there is some sort of error in the ingredients list of your recipe if it is asking for curry powder in addition to mustard seeds, tumeric, and cumin. I might add the mustard seeds, but I'd just ignore the additional tumeric and cumin.

Do people in India use curry powder? - NO
curry powder is the mix spice powder used in many non Indian foods to give Indian flavor.
so you will never see curry powder used in Indian recipes by Indian chefs

curry powder is milder version of Garam Masala with more of cumin and coriander seeds
recipe for garam masala
http://www.vahrehvah.com/garam+masala+pwr:3964

www.vahrehvah.com

As others have stated, it depends on the specific brand of curry powder you're using.

Further, it's not uncommon for recipes to call for additional cumin etc because the proportion needed for that specific recipe may be different than the balance in the curry powder.

Cumin, yes. Turmeric, no.